The invention relates to carrier current systems in which digital data is transmitted over AC power lines in the form of modulated "carrier current" signals to accomplish digital communication between two separated locations in an environment in which there is a great deal of spurious noise, especially in the form of large, long duration "impulse noise" signals on the AC line (or any other line in a high noise environment).
The general concept of communicating digital signals from one place to another by superimposing them on an ordinary AC power line is well known. Many specific implementations of this concept have been proposed and/or augmented. Along with the explosive and pervasive growth of using computers to control and/or communicate with various devices in commercial buildings (and even in residences), and in view of the high cost of installing the necessary multi-conductor data buses in buildings to allow such data communication, there has been considerable effort devoted to developing carrier current transceiver (CCT) devices to allow date communications via the ordinary AC power lines in such buildings. Unfortunately, in most commercial establishments, such as restaurants, commerical offices, manufacturing operations, etc., in which there is a need for rapid digital data communication throughout a building, there is an extremely high level of spurious, high magnitude, long duration noise signals on the AC power lines. This noise constitutes a hostile environment which has led to the requirement that CCT systems be implemented with complex, expensive circuitry that prevents the various noise signals from being interpreted as data, or prevents data signals from being drowned out by the noise signals. High noise signal levels and widely varying AC line impedances in a typical AC power line of a typical commercial establishment are caused by a variety of varying loads that are placed on the AC power line. Such loads include the various electrical motors of numerous electrical appliances, light dimmer circuits, filters of television sets, etc. "Impulse noise" pulses having durations in excess of 500 microseconds and having repetition rates that may be substantially in excess of the 60 hertz power line frequency and containing a wide spectrum of frequencies, including frequency components that easily pass through bandpass filters in CCT circuitry are not uncommon on the AC power lines of some commercial establishments. The known CCT circuit techniques sometimes do not attenuate the "noise frequency" components of impulse noise sufficiently to prevent the noise frequency components from being misinterpreted as actual data. Some prior carrier current transceiver devices attempt to transmit digital pulses directly over the AC power line. However, in environments wherein there is a high level of the above-mentioned impulse noise, these digitals pulse techniques have serious shortcomings, since impulse noise is likely to completely dominate some of the digital pulse signals and prevent them from being detected. Unless complex and expensive data recovery techniques are used, this type of data communication will be impaired. Such digital pulse transmission techniques generally require transmission of rather high magnitude pulse signals, which inevitably generate harmonic frequency components, commonly referred to as RFI (radio frequency interference). This RFI frequently interferes with radio and television communications, and is subject to FCC regulation. Furthermore, if numerous digital pulse type carrier current transceivers are to be coupled onto the same AC power lines, complex and expensive encoding and decoding techniques must be utilized to ensure that the different CCT pairs do not interfere with each other. Perhaps to overcome these difficulties, frequency domain techinquies have been utilized to accomplish carrier current communications via AC power lines in commercial establishments, as shown in U.S. Pat. Nos. 3,659,280 and 4,057,793. Other prior art references which are believed to be representative of the state of the art include U.S. Pat. Nos. 3,451,052; 3,460,121; 4,040,046, 4,075,675; 4,270,206; 4,302,844 and 4,308,619. The most sophisticated known prior approach to carrier current transceiver communication is suggested in a paper entitled "A Carrier Current Transceiver IC for Data Transmission Over the AC Lines", which was presented by National Semiconductor Corporation at the 1982 IEEE International Solid State Circuits Conference at Session XII (Data Communications) at Santa Clara, Calif. This paper briefly describes circuitry of a proposed monolithic integrated circuit chip that allegedly functions reliably in communicating "frequency domain" digital data along an AC power line with a high level of electrical noise thereon. The disclosed circuit uses two low frequencies to modulate a carrier to produce on an FSK (frequency shift keyed) data format to represent binary "1"s and binary "0"s. However, the circuit allows impulse noise at either of the shifted carrier frequencies to pass through the filters or tuned circuits and then relies on time domain filters to recover the real data. This proposed circuit is quite complex and would not be reliable at high data rates in circumstances in which the impulse noise is of long duration relative to the highest permissible data rate. The circuitry utilized for generating the FSK signal would have a tendancy to be somewhat unstable with respect to temperature or else would probably have to use two crystal oscillators and would be quite expensive. This technique would tend to increase the error rate due to temperature drift.
Other prior art seems to lead to the conclusion that in high noise environments, tone modulation and/or use of automatic level control circuitry are necessary in order to reliably communicate digital data at relatively high data rates on a typical AC power line of a commercial establishment. (Tone modulation is the use of a low frequency signal to modify or modulate a single carrier so that it represents either a "1" or a "0" in contrast to FSK techniques, which use two different carrier signals of different, but usually closely spaced frequencies to represent a "1" and a "0"). This leads to higher complexity and higher costs than is desirable.
Some carrier current communication systems synchronize the carrier current with the 60 Hz frequency of the AC power line in attempting to avoid noise impulses thereon. This approach adds cost and complexity, and is not very effective if cheap light dimmers or other switching devices which do not synchronize their switching with the 60 Hz line frequency are not used, or if multiple phases are used in the AC power system.
Frequently, it is advantageous to encode digital data that is sent over a communications medium. For example, it may be advantageous to encode the data in such a way that clocking information is included with the data. It is desirable that the circuitry in a carrier current communication system be able to properly transmit and receive data that is to encoded.
Accordingly, it is an object of the invention to provide an improved carrier current transceiver system and method which is capable of operating at relatively high data rates in a system wherein the AC power line has large quantities of high magnitude, long duration impulse noise.
It is another object of the invention to provide a carrier current transceiver system and method which is capable of detecting the presence of a carrier current signal in the presence of long duration impulse noise.
It is another object of the invention to provide a low cost, highly reliable, high speed carrier current transceiver system and method for operating in high impulse noise environments.
It is another object of the invention to provide a carrier current transceiver system and method which is highly resistant to damage caused by large magnitude noise signals on the AC power line.
It is another object of the invention to provide an improved, relatively simple, inexpensive, highly stable carrier current transceiver system and method which is highly stable with respect to temperature and which reliably produces a signal representing a digital logic state transmitted over the AC power line in the middle of a bit cell minimum strobe width pulse window of a data recovery circuit.
It is another object of the invention to provide a system which does not require synchronization of the data to the power line frequency and thereby avoids the complexity of this approach and the limiting of the maximum data rate that results.
It is another object of the invention to provice a system which does not limit the format or encoding of data presented to it.
It is another object of the invention to provide an improved carrier current transceiver system and method which substantially increases the number of transceiver units that can be coupled to a particular AC power line without producing so much loading on the AC power line that carrier current signal levels are reduced to an unacceptable level.